In the semiconductor device manufacturing, a photolithography process is frequently used in reproducing a circuit onto a wafer. In order to transfer an image onto the wafer surface, i.e., an image of an IC circuit, a photomask must be used.
A mask for photolithography is usually constructed on a transparent plate, called a blank, covered with a patterned film of opaque material. The blank may be made of soda lime glass, borosilicate glass or fused quartz. In advanced photolithographic technology, fused quartz is more frequently used for its advantages of being transparent to deep UV, and for its low thermal expansion coefficient. The low expansion coefficient is important when the minimum feature size is less than 1.5 .mu.m. Furthermore, distortions related to thermal expansion become more pronounced as the mask size is increased. As a result, quartz masks are recommended for exposures onto 150 mm and larger wafer sizes and for minimum feature sizes of smaller than 1.5 .mu.m.
The quartz material is selected also for its low sodium content, its high chemical stability and its high light transmission character. On top of the quartz plate, an opaque material having typically a thickness smaller than 100 nm is formed of chrome, and covered with an anti-reflective coating film such as chrome oxide to suppress interferences at the wafer surface. A suitable thickness for the chrome layer is normally 1,000 .ANG., while a suitable thickness for the chrome oxide layer is normally 200 .ANG..
High quality photomasks must meet stringent property requirements such as flatness, accuracy of pattern placement, minimum feature size, linewidth control over the entire area of the mask, and defect density. A small variation in the flatness can alter the optical path length and thus cause large distortions on the resist due to defocusing. Temperature variations occurred during mask fabrication and during the mask use account for most of the misplacement of the pattern and misregistration between masking levels. It has been found that, for example, a 1.degree. change in temperature in the mask-making process can cause a misplacement of approximately 0.1 .mu.m over a 100 mm diameter area on a quartz mask.
The thermal stability of the quartz mask, including that of the chrome film coated on the mask, is therefore an important factor in achieving a successful photomasking process. To ensure the maximum thermal stability of a quartz mask is achieved, a post-baking process is normally carried out after the mask making process. A typical post-baking apparatus 10 that is commercially available is shown in FIG. 1. In the post-baking apparatus 10, a chamber 12 is provided which consists of an upper plate 14 mounted in an upper plate mounting frame 16 and surrounded by sidewalls 18, enclosing a bottom plate 20. The bottom plate 20 is provided with a plurality of apertures 24 along a peripheral edge of the plate for flowing an inert gas therethrough. A suitable inert gas used is nitrogen. The bottom plate 20 is heated by electrical heating means (not shown) that is embedded in the plate body. A nitrogen inlet 28 is further provided for flowing a nitrogen into the plurality of apertures 24 in the bottom plate 20. The upper plate 14 is used as a heat reflector and has an exhaust port 32 formed at a center of the plate for exhausting the heated inert gas such that it may be recirculated.
A detailed, exploded view of the heating chamber 12 of the post-baking apparatus 10 is shown in FIG. 2. The upper plate, or the heat reflector plate 14 is supported at four corners by insulating support posts 34 which may be suitably made of a ceramic material. On top of the bottom plate 20, a substantially flat object 30 such as a photomask is supported by support pins 36. A robot blade (not shown) is used for loading and unloading the photomask 30 into and out of the heating chamber 12. A plurality of photosensors 38 are further provided for determining the post-baking temperature, which is normally higher than 100.degree. C.
A more detailed view of the interior components of the heating chamber 12 is shown in FIGS. 3, 4 and 5. FIG. 3 illustrates an enlarged, cross-sectional view of the upper reflector plate 14, the bottom plate 20, the retractable support pins 36 and the photosensors 38. It is seen that a substantially flat object 30, such as a photomask is positioned on top of the bottom plate 20. An enlarged, partial view of the same components is shown in FIG. 4 indicating a minimum distance maintained between the heat reflector plate 14 and the photomask 30 is X which may be suitably determined depending on the characteristics of the photomask.
In the configuration of the heat reflector plate 14 shown in FIGS. 3, 4 and 5, the post-baking apparatus used for mask making cannot achieve thermal uniformity during the post-baking process. The temperature non-uniformity comes from the low thermal conductivity of quartz and its relatively large thickness of approximately 0.25 in. It has been found that, in a conventional post-baking apparatus, the temperatures achieved at the center of the photomask and at the corners of the photomask deviate by approximately 5.degree. C. Such temperature gradient is sufficient to cause a large difference in the material properties in subsequent processes of the photomask. The large temperature differential is caused by the fact that before a photomask reaches a steady state temperature condition, heat transfer at the corners is significantly more than at the center due to larger heat transfer area available at the corners.
It is therefore an object of the present invention to provide an apparatus for heat treating a substantially flat object that does not have the drawbacks or shortcomings of the conventional apparatus.
It is another object of the present invention to provide an apparatus for heat treating a substantially flat object that does not have the drawback of forming a temperature differential on the surface of the flat object during heat treating.
It is a further object of the present invention to provide a post-baking apparatus for heat treating a substantially flat object by utilizing a specially designed heat reflector that is positioned juxtaposed to a top surface of the flat object.
It is another further object of the present invention to provide a post-baking apparatus for a substantially flat object that is equipped with an upper heat reflector that has edge portions curved upwardly away from the object to be heat treated.
It is still another object of the present invention to provide a post-baking apparatus for heat treating a substantially flat object which is equipped with an upper heat reflector plate that has edge portions curved upwardly forming an angle between about 15.degree. and about 45.degree. from a horizontal plane.
It is yet another object of the present invention to provide a post-baking apparatus for heat treating a quartz photomask by positioning the photomask in a heating chamber adjacent and opposite to a heat reflector plate that has edge portions curved upwardly.
It is still another further object of the present invention to provide a hot plate heating apparatus for post-backing a quartz photomask that is capable of heating the photomask and achieving an uniform temperature across the mask surface.
It is yet another further object of the present invention to provide a hot plate heating apparatus for post-baking a quartz photomask by utilizing an upper heat reflector plate that has edge portions curved upwardly such that more uniform reflected heat is available for heating the photomask positioned therebelow.